2011-26-0119

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J S Karajagikar, N R Rajhans, B B Ahuja College of Engineering, Pune, IndiaR G RajhansTata Technologies Ltd., Pune, India

Vibration Analysis on Driver Seat for Small Cars

ABSTRACTIn India, small car segment is having maximum sale, which includes cars like Maruti 800, SUZUKI Swift, Maruti Alto, Tata Indica, etc. Driver seat is one of the main aspects to be considered while defi ning comfort in a moving vehicle. The current analysis concentrates on driver seat because driver comfort is of main concern since it is the most occupied seat in any vehicle and the occupancy is for longer duration. In addition to sitting, the driver job is to manipulate different controls and concentrate parallely on many aspects. The research work aims at studying the vertical vibrations transferred to the human body via seat.

The work is an attempt towards studying dynamic characteristics of driver seat for comfort through objective evaluation. For objective evaluation, two tests were conducted; Seat Effective Amplitude Transmissibility (SEAT) test and Ride Comfort Index test under two different conditions i.e. car level and seat level testing on Car ‘A’ and Car ‘B’. Both tests were carried out under controlled conditions.

Car level test was carried out on 4-Poster simulator, which can simulate exact road conditions in vertical direction whereas seat level testing is done on electro-dynamic shaker, which simulates sinusoidal signals. Results of experimentations i.e. accelerations at seat base and seat mount were compared with reference standard ISO 2631-1(1997) which indicated average Root Mean Square (RMS) of Car ‘A’ and Car ‘B’ is 0.007 and 0.008 Km/s2 respectively. Hence according to ISO 2631-1(1997), both cars fall into ‘Fairly uncomfortable zone’. SEAT Test revealed transmissibility in two different positions (Front most and rearmost position). In front most position, transmissibility is 65.66% and 204% for Car ‘B’ and

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2011-26-0119Published on19th-21st January 2011 SIAT, India

Car ‘A’ respectively, whereas transmissibility is 63.56% and 145% for Car ‘B’ and Car ‘A’ in rearmost position of seat.

Keywords : Ride Comfort, Driver Seat, Four-Poster Simulator, Transmissibility, Comfort Index

INTRODUCTIONHertzberg [1] defi nes ride comfort is absence of discomfort. Ride comfort analysis is one of the important aspects of the vehicle from performance point of view. For objective evaluation of Ride comfort, two tests were conducted namely SEAT% [2] (Seat Effective Amplitude Transmissibility) test and Ride Comfort Index [3] test under two different conditions i.e. car level and seat level testing and their comparison with benchmark car. Both tests had been carried out under controlled conditions (Experimental works in laboratory were considered as controllable) and results were correlated with reference standard ISO 2631–1(1997).

SEAT % TEST (COMPONENT LEVEL TESTING)SEAT % experimental analysis carried out on driver seat is to evaluate gains, seat effective amplitude transmissibility, and attempted to understand seating systems’ damping characteristics under steady state conditions. During laboratory tests, Shaker Machine is used to generate sinusoidal signal. Due to safety factor, a mass system with dead load weighted about 50kg (which is about the sitting weight of an average-size person on a seat) was used in the laboratory dynamic test. This test was carried out by increasing the frequency, from 5 Hz until 100 Hz.

Symposium on International Automotive Technology 2011

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The instrumentation and set-up used for the test is as follows (Fig. 1):1. Shaker universal testing machine,2. Tri-axial accelerometers: 3 Nos. � Positions 1: Shaker table, � Positions 2: Seat mount, � Positions 3: Seat Base.3. Data acquisition unit,4. Post processor,5. Rigid dummy: 50kgs (equivalent to seating weight)

Frequency Weighing for Driver Seat:

With ref. to ISO 2631-1 (Table 1), frequency weighing Wb(f) for Car ‘A’ and Car ‘B’ seat in “Z” axis is considered as “Wk” =1.

Figure 1. Shaker Test Set-up

Input data was obtained from the accelerometer installed at the seat mounts, whereas output data would be obtained from the seat pad accelerometer put on the seat base. The acceleration results are listed as per different experimental conditions. The test was carried out on seat with dummy loaded on it. Output graphs were in terms of accelerations vs time (time domain output), accelerations vs frequency (frequency domain output), and transmissibility vs frequency.

Seat % OutputSeat values were obtained for car ’A’ and car ’B’ driver seat by importing the post-processed (power spectrum) data from the analyzer into the Microsoft Excel program. The “ride on the seat” (output) is the integral of frequency-weighted experienced on the seat, whereas the “ride on the fl oor” (input) is the integral of frequency-weighted experienced on the fl oor. From the basic knowledge of integral, this equation can be stated as the ratio of the area under the graph of “ride on the seat” to the area under the graph of “ride on the fl oor”, as below:

(1)

Where, Gs (f) : Accelerations at seat base (output),

Gf (f) : Accelerations at fl ooring (input),

Wb (f) : Frequency Weighing factor.

Table 1. Weighing Factors as per Location & Axis of Measurement [3] - ISO2631-1[1997]

Evaluation of Dominant Frequencies and Transmissibility (Gains) Analysis

Using the set-up explained above and input as sinusoidal signal (Acceleration: 1G, Frequency swept: 5Hz-100Hz, frequency sweep interval: 1Hz) acceleration at the shaker table was given and response at different positions of seat was found out. Result determined the dominant frequency and gains (transmissibility) between different points of seat for Car ‘A’ and Car ‘B’.

Fig. 2 shows response for Car ‘A’ – Rear seat Position and Fig. 3 shows response for Car ‘A’ – Front seat Position and Fig. 4 shows response for Car ‘B’ – Rear seat Position and Fig. 5 shows response for Car ‘B’– Front seat Position.

Dominant frequencies in both cases were found out which is around 5 Hz. Transmissibility (SEAT %) for car ‘A’ and Car ‘B’ in both seat positions (rear and front) was calculated, (Table 2 and Fig. 6)

Figure 2. Frequency Domain Data: Car ‘A’ Seat Position: Rear


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The SEAT % test reveals that Car ‘B’ seat is more comfortable than the Car ‘A’ seat since Car ‘B’ seat is having better vibration damping properties than car ‘A’. I.e. Transmissibility of Car ‘B’ seat is less than 100% whereas transmissibility of Car ‘A’ seat is more than 100%.

DYNAMIC SEAT TEST ON 4 POSTER SET-UPDynamic tests were carried out on 4 Poster simulator which is used to simulate exact road profi le and conditions in the vertical directions. During the test, sinusoidal offset signal and random signal were generated by 4 poster simulator. In this test, a subject weighted about 50kg was used in the laboratory dynamic test.

The instrumentation and set-up used for the test is as follows (Fig. 7):1. Four poster simulator,2. Tri-axial accelerometers: 3 Nos. a. Positions 1: Shaker table, b. Positions 2: Seat mount, c. Positions 3: Seat Base.

Figure 3. Frequency Domain Data: Car ‘A’ Position: Front

Figure 4. Frequency Domain Data: Car ‘B’ Seat Position: Rear

Figure 5. Frequency Domain Data: Car ‘B’ Seat Position: Front

Figure 6. Car ‘A’ & ‘B’- SEAT % vs Seat Locations

Table 2. Results- Car ‘A’ & ‘B’ - SEAT % vs Seat Locations

Figure 7. Four Poster Set-up


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3. Data acquisition unit,4. Post processor,5. Rigid dummy: 50Kgs

This test was carried out by increasing the frequency, from 1 Hz until 50 Hz taking input as the road profi le for both car ‘A’ and car ‘B’. There were two kinds of data in the test; input data and output data. Input data was obtained from the accelerometer installed at the seat mounts, whereas output data would be obtained from the seat pad accelerometer put on the seat base.

Evaluation of Transmissibility (Gains) and Dominant FrequenciesUsing the set-up explained above and input as sinusoidal signal (amplitude: 5mm, Frequency swept: 1Hz-50Hz, frequency sweep interval: 1Hz) acceleration at the four poster was given and response at the different positions of seat was found out. Result determined the dominant frequency and gains (transmissibility) between different points of seat for Car ‘A’ and Car ‘B’.

Seat response for Car ‘A’ and Car ‘B’ is shown in Fig. 8 and 9 respectively. The transmissibility and natural frequency of seat ‘A’ and ‘B’ are determined using this response and its adverse effect on human body is indicated in Table 3 and 4 for Car ‘A’ and Car ‘B’ respectively.

Combined RMS (av) indicated in Table 5 is used to fi nd out Ride Index as per ISO2631-1 (1997) as shown in Table 6 and calculated using following relation,

(2)

Where, awx : Weighted RMS acceleration w.r.t. axis x, awy : Weighted RMS acceleration w.r.t. axis y, awz : Weighted RMS acceleration w.r.t. axis z, av : Combined R.M.S., k : Multiplying factors.

Figure 8. Car ‘A’ Seat Response: (Road Profi le 1-50 Hz)

Figure 9. Car ‘B’ Seat Response:(Road Profi le 1-50 Hz)

Table 3. Results for Vehicle Level Testing for Car ‘A’

Table 4. Results for Vehicle Level Testing for Car ‘B’

Table 5: Results for Ride Comfort Index Vehicle Level Testing for Car ‘A’ and Car ‘B’


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Road profi le simulation illustrated indicates that, Ride Index for Car ‘A’ is better than Car ‘B’ which means Car ‘A’ as a whole system (vehicle level) is better as compared to Car ‘B’.

CONCLUSIONSObservations based on the experimentations conducted on Car ‘A’ and Car ‘B’ are:

� In seat level testing, SEAT value indicates, Car ‘B’ seat damps the vibration whereas Car ‘A’ seat amplifi es the vibrations at seat top.

� In vehicle level testing, overall vehicle is considered for testing vibrations in vertical direction. According to ISO2631-1(1997), the Ride Index for both cars ‘A’ and ‘B’ are in ‘Fairly Uncomfortable’ zone.

REFERENCES1. Hertzberg H T E, “The Human Buttocks in Sitting:

Pressures, Patterns, and Palliatives”, SAE Paper, No. 72005, 1972

2. IS14917 (Part-1):2001, ISO10326-1:1992, Mechanical Vibration – Laboratory Method for Evaluating Vehicle Seat Vibration – (Part 1 Basic Requirements)

3. ISO 2631-1 (1997): Mechanical Vibration and Shock – Evaluation of Human Exposure to Whole Body Vibration – Part 1: General requirements. (Page No. 14 -16)

4. BA7054 Standard: Human Vibrations

ACKNOWLEDGMENTSI sincerely thanks to all those who have supported me to carry out this work.

CONTACTJayant S. KarajagikarE-mail: [email protected]

DEFINITIONS1. SEAT % [2]: Seat Effective Amplitude Transmissibility

(SEAT) is a non-dimensional measure of the effi ciency of a seat in isolating the body from vibration or shock. SEAT values have been widely used to determine the vibration isolation effi ciency of a seat. SEAT% value is defi ned as:

2. Ride Comfort Index [3]: It is the qualitative measure of comfort as per reference standard ISO-2631-1[1997].

Table 6. Ride Comfort Index [3]

The Technical Paper Review Committee (TPRC) SIAT 2011 has approved this paper for publication. This paper is reviewed by a minimum of three (3) subject experts and follows SAE guidelines.

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2011-26-0119

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